1. Field of the Invention
This invention is directed to lithographic printing plates comprising a hydrophilic polymer containing thiosulfate groups. More particularly, this invention is directed to a method to reduce the imaging effluence of printing plates comprising a thiosulfate polymer.
2. Background Information
One approach to process-on-press printing plates includes the use of polymers containing thiosulfate groups, which cause the polymer to be water soluble or water dispersible. Upon heating, chemical and/or physical changes to the polymer cause it to become water insoluble. Thus, when such polymers are used in the coating of a lithographic printing plate in conjunction with a photothermal converter such as carbon black or an IR dye, the plate may be imaged with an IR laser and then placed directly on the press without an intermediate processing step. The coating in the non-imaged areas is removed by the fountain and/or ink of the press while the coating in the imaged areas is retained. The retained coating is oleophilic while the substrate exposed upon coating removal in the non-imaged areas is oleophobic.
A disadvantage to this approach is that there may be an objectionable effluence upon imaging the plate due to the presence of the thiosulfate groups. The term effluence as used herein is intended to mean an odor that flows out of the printing plate or out of the printing plate coating.
Processless printing plates, which do not require a chemical development step, have been described in U.S. Pat. No. 5,922,512, U.S. Pat. No. 6,040,115, U.S. Pat. No. 6,190,830, U.S. Pat. No. 6,190,831, and U.S. Pat. No. 6,261,740, which disclose imageable elements containing heat-sensitive polymers. The heat-sensitive polymers provide imaging means without wet processing. None of these patents disclose thiosulfate-containing polymers.
Printing plates comprising thiosulfate-containing polymers have been described in U.S. Pat. No. 6,162,578, U.S. Pat. No. 6,146,812, U.S. Pat. No. 6,136,503, and U.S. Pat. No. 5,985,514. The imaging layer in these patents is the only layer. None of the patents describe printing plates that include topcoats or printing plates that include diazonium, iodonium, copper(I), alkoxypyridinium or maleimide additives.
WO 98/55311 is directed to a lithographic printing plate that contains a metallic imaging layer. The plate works by ablation of the metallic layer from the aluminum support. The topcoats include polyvinyl alcohol, gum arabic, polyvinylphophonic acid, carboxymethylcellulose, and polyethylene glycol. The patent does not describe an imaging layer containing a thiosulfate copolymer.
U.S. Pat. No. 5,939,237 is directed to an article mounted on a press. The article comprises a photosensitive composition. The patent does not describe a thermal printing plate. The hydrophilic areas of the coating are not designed to be washed off.
U.S. Pat. No. 5,506,090 is directed to a photosensitive composition. The patent does not describe a thermal printing plate.
JP 2000 071635 describes a printing plate containing a thermally sensitive layer. The imaging layer is hydrophobic and becomes hydrophilic with heat. The thermally sensitive layer does not comprise a thiosulfate-containing polymer.
JP 2001 162962 describes a printing plate with a topcoat laminated onto the plate. The topcoat contains an IR dye, while the bottom coat does not. The patent is not directed to reducing or eliminating effluence from a printing plate. The thermally sensitive layer does not comprise a thiosulfate-containing polymer.
None of the methods in the prior art provides a method to reduce effluence during the imaging of printing plates comprising thiosulfate-containing polymers by application of a water soluble top-coat or by using formulations containing diazonium, iodonium, copper(I), alkoxypyridinium or maleimide additives.
We have found that application of a water soluble top-coat or the use of a coating composition comprising a photothermal converter and diazonium, iodonium, copper(I), alkoxypyridinium or maleimide additives to the plate significantly reduces or eliminates the objectionable effluence that normally arises upon imaging of a printing plate comprising thiosulfate-containing polymers. The present invention has the advantage of allowing the preparation of printing plates comprising thiosulfate-containing polymers that become insoluble upon thermal imaging while reducing or eliminating the effluence associated with such polymers.